An electro-optical sensor is a device that converts radiant energy of a particular wavelength or range of wavelengths into an electrical signal. One example of an electro-optical sensor is a handheld TV camera, which converts images of visible light into an electrical signal that can be stored in electronic memory or displayed on a television screen. Another example of an electro-optical sensor is a Forward Looking Infrared sensor ("FLIR"), which converts images of infrared energy (i.e., heat), invisible to the human eye, into an electrical signal that can be displayed to a viewer. Because the FLIR can "see at night," it is widely used in military navigation and targeting systems and more recently in commercial security systems.
The capability of the electro-optical sensor to resolve the details of an object is referred to as the "resolution" of the sensor. Increasing the resolution allows the sensor to identify small objects at greater distances. High resolution allows the FLIR to identify an enemy target before that target gets too close.
Shading and offset diminish the resolution of the electro-optical sensor and, therefore, must be removed. When the target is small, the sensor's electronics must increase the gain and brightness to a high level in order to see the necessary detail in the target. Although the details of the target become more visible, the background becomes distorted by internal reflections and temperature gradients which result from the increases in gain and brightness. Offset refers to the average level of brightness (temperature) over an entire visible (infrared) sensor, and shading refers to gradients in brightness (temperature) across the sensor. These distortions make it harder to distinguish the target from the background.
Shading and offset cause additional problems when the resolution of the sensor is being measured. A standard measurement for sensor resolution is the Modulation Transfer Function ("MTF"). During MTF measurement, a very small target (e.g., a slit target) is presented to the sensor, and data samples of the sensor's video signal are collected. After all of the samples have been collected, shading and offset are manually removed from the samples and Fourier analysis is performed. This procedure requires an operator to manually derive the offset and shading from trends in data, and then subtract the offset and shading from the data.
Operator intervention during the MTF test is undesirable. The operator must have experience and training to determine trends in the data. Even then, the determinations are subjective and prone to error. Moreover, operator time is required to determine the trends, preventing the test results from being available in real-time.
It is an objective of the present invention to remove shading and offset from a video signal.
It is another objective of the present invention to increase the repeatability and accuracy of an MTF measurement.
It is yet another objective of the present invention to increase the speed and lower the cost of performing an MTF measurement.